I the context of music and it's history, Nirvana was good. A breath of Fresh air into a stale and oversaturated music scene. Cock Rock was on the decline and the fuck you attitude of the punk era has been long underground. The response to this oversaturation of prestine solos, and meaningless music came the stripped down "garage rock" as seen in the 70's with Iggy Pop and similar bands.
Nirvana was the reinvention of that with the soul that touched millions of then alienated teens, who couldn't identify with the booze and bitches of the cock rock.
People wanted music with emotion and some sense of purpose in the lyrics. They wanted art not commercial and that is exactly why Nirvana is good. People who don't understand the history of music and comment like they know something are fucking idiots.
I'm right because I have a degree, an all you kids don't.
But in the context of METAL then yes Nirvana may verywell suck simply because metal ins't about feeling as much as it is about aggression and technical skill. But Nirvana isn't metal and therefore metalheads saying that Nirvana sucks is about as valid as Adult Contemporary musicians saying that metal sucks.